CMOS image sensors are used widely, for example, in digital cameras and night vision goggle (NVG) devices. When exposed to light, the CMOS image sensor captures an image. The image sensor typically includes a large array of pixels that are organized into rows. There are times when the pixels in the array are not all exposed to light at the same time. Rather, the pixels are exposed sequentially, row by row. This method is known as a rolling shutter. The exposure time for a single row of pixels is called the exposure period. The total time required to expose and process the pixels in the entire array is known as the frame period.
Another method of capturing an image is known as a global shutter, or a snapshot operation. In this method, the start and end of integration for all rows in the imager is the same. Typically, at the end of integration, the pixel values are moved to storage capacitors to be read out while the next integration cycle begins.
One problem associated with the rolling shutter method is that the illumination level of the light source may vary over time. This variation is called flicker. When exposed to flicker, an image sensor may capture the flicker as bands of contrasting brightness in the final image. When exposed to very bright light, the final image may be overexposed.
FIG. 1 shows a basic three-transistor pixel circuit 101 used in prior art image sensor arrays. A transistor M1 connects a cathode (node 103) of a photodiode 125 to a voltage supply, Vdd 107. The anode of photodiode 125 is connected to ground. The gate of transistor M1 is connected to a reset signal 109. Transistor M3 connects Vdd 107 to another transistor M5. The gate of transistor M3 is connected to node 103. The gate of transistor M5 is controlled by a row select signal 111, while its source is connected to a column output line 113, from which the output of pixel circuit 101 is read. Transistor M3 is used as a source follower to buffer photodiode 125 and prevent it from being loaded down by column output line 113.
During operation, photodiode 125 is reset to the supply voltage Vdd 107 at the beginning of an exposure period, by asserting reset signal 109 and charging node 103. As photodiode 125 is exposed to incident light, it accumulates more charge and the voltage at node 103 decreases. The voltage across photodiode 125 is indicative of the light intensity that photodiode 125 has been exposed to over time. At the end of the exposure period, row select signal 111 is asserted to read out the values of a row of pixels in the image sensor array.
In operations using electronic image intensifiers (EI2) the photodiode is not used. Charge is directly accumulated into the pixel capacitance, as it is “seeing” electrons not photons.
Pixel circuits are generally designed to improve pixel sensitivity under low-light conditions. However, if lighting conditions are too bright, the photodiode accumulates too much charge and reaches saturation, at which point the voltage at node 103 falls to zero. Further exposure of the photodiode cannot be registered, because the voltage cannot fall below zero. As a result, the output signal of the pixel is clipped, and the final image looks overexposed.
FIG. 2 shows two different transfer curves for the conversion gain of a pixel, such as that shown in FIG. 1, assuming that its gain could be varied. The figure plots the signal output of a pixel versus the intensity of the incident light during an exposure period. The saturation level of the photodiode in the pixel is indicated by dotted line 305. The line 307 is a transfer curve with one level of sensitivity, which clips at a low light intensity level. The line 309 is another transfer curve which provides lower sensitivity, but does not clip as early as line 307. It is desirable, therefore, to increase the dynamic range of a CMOS imager by preventing its pixels from saturating and clipping the received light intensity.
The dynamic range of a CMOS imager is further complicated when considering night vision goggle (NVG) systems. Referring to FIG. 3, there is shown an NVG system, designated generally as 30. The NVG system includes photocathode 31, multi-channel plate (MCP) 32 and CMOS imager 33. The light, as photons, are received by photocathode 31 and converted into electrons. The electrons are amplified by MCP 32 and sent to electron-sensing CMOS imager 33. The CMOS imager includes control and processing electronics (not shown) for providing a processed digital video output to a user.
Two control signals that are pertinent to the present invention are shown functionally in FIG. 3. As shown, a gated signal turns ON/OFF the photocathode, thereby acting as a shutter control for the photocathode. When the gated signal is ON, the photocathode permits received light to pass through the photocathode and be transmitted as electrons toward the CMOS imager. When the gated signal is OFF, however, the photocathode acts as a closed shutter and prevents light transmission to the CMOS imager.
The other control signal is referred to herein as Vresetlow, which sets a threshold voltage level in the CMOS imager, so that any light intensity above the set threshold voltage level is clipped. The operation of this control signal is explained by referring to FIGS. 4 and 5.
Referring first to FIG. 4, each pixel includes two integration periods, referred to herein as integration1 and integration2 (also referred to as t1 and t2). It will be understood that the duration of each integration period may be varied. For explanation purposes, FIG. 4 shows the period of integration1 as 15 msec; and the period of integration2 as 1 msec. Thus, to fully charge a pixel, an integration period of 16 msec is required, which includes integration1 and integration2.
At the pixel level of the imager, the integrated charge on the pixel has a predetermined set threshold level that the charge cannot exceed during the first period of the integration time (integration1). In the example shown in FIG. 4, the charge cannot exceed 3000 ADUs during the first period. During the second period (integration2), however, the Vresetlow is removed, so that the pixel is able to continue integrating, until a full charge is obtained at 4095 ADUs. It is assumed in the example that the full charge is 4095 ADUs and that the first integration period cannot charge above 3000 ADUs. It will be appreciated, however, that these ADU levels may be different and may be set to other levels.
The above described approach is known as a variable well. At the first integration period (for example 15 msec), the well of the pixel cannot charge above a set threshold (for example 3000 ADUs). During the second integration period (for example 1 msec), the well of the pixel is permitted to charge up to its full well capacity of 4095 ADUs (for example), designated as 41 in FIG. 4.
As demonstrated by curve 42 in FIG. 4, the pixel has a relatively bright input, so it quickly reaches the set threshold of 3000 ADUs. The pixel is held at that charge until 15 msec expires. The pixel is released after the 15 msec period to integrate up to its full well capacity. This helps the pixel in preventing saturation under high illumination conditions and, thereby, increases the dynamic range of the incoming light that the imager may capture. This is also referred to herein as a high dynamic range (HDR).
A different phenomenon, however, may be seen by examining curve 43. As shown, the pixel integrates during the first integration period. Because the light is not as bright, as compared to the light seen by the pixel integrating under curve 42, the pixel never reaches the Vresetlow threshold of 3000 ADUs. During the second integration period, the pixel is released to integrate again and continues to charge until the end of the frame period.
Similarly, upon examining curve 44, dark areas of an image never reach the Vresetlow threshold (3000 ADUs, for example). The pixel continues to integrate normally, as shown by curve 44.
Referring now to FIG. 5, there is shown an exemplary method for reading out pixel intensities during a rolling shutter operation. The example assumes that there are 1024 rows (also referred herein as lines) in the pixel array of the CMOS imager. As shown, the readout of frameN-1 and frameN assumes a readout period of 16.67 msec (60 Hz image). Each frame is gated ON/OFF by a gated signal acting as a shutter on the photocathode. Line 1 of the pixel array is gated ON/OFF, as shown. Line 2 of the pixel array is gated ON/OFF at the same time as line 1, but is read out approximately 16 μsec later due to the rolling shutter operation. This delay is incurred by a line rate of approximately 62 kHz (1/line rate equals approximately 16 μsec). By the time line 512 is read out, during the rolling shutter, a delay of approximately 8.2 msec is incurred (512×16 μsec). By the time line 1024 is read out, during the rolling shutter, a delay of approximately 16.4 msec is incurred (1024×16 μsec).
Thus, as shown, each pixel integrates during frameN-1 and frameN. As the shutter rolls, each line has its integration time delayed by one row (approximately 16 μsec). In a 1280×1024 pixel array and using a 90 MHz clock to is read each pixel, it takes approximately 11.11 μsec to read each pixel. Therefore, as an example, each frame is read out in approximately 16 msec (1280×11.11 μsec equals approximately 16 msec).
When a variable well approach is added to a night vision device including a gated photocathode, the timing interaction between one frame and the next frame becomes more significant. Furthermore, when a rolling shutter approach is used (as compared to a global shutter approach), the timing interaction becomes even more significant. As will be explained, the present invention provides an improvement in the dynamic range of a CMOS imager, when all three of the above timing events are involved. In other words, when the CMOS imager's performance is based on (1) the line integration time, (2) the timing of the variable well's break-point during integration, and (3) the duty cycle of the gated pulse of the photocathode, the present invention provides an increased dynamic range for such a CMOS imager.